Opening created from tank failure

Corrosion Science and Engineering

Helping to ensure

• the environment

• worker safety

• and product quality

Objectives of Training

• Participants will be able to recognize “Corrosion” issues before machinery or process equipment is designed or installed.

• Avoid business issues (liability, product disruption, product quality, cost of equipment replacement) related to engineering materials and design.

Expected Outcomes of Training

• Engineers will become better at selecting materials of construction and designing structures that avoid corrosion.

• R&D Product Development will become aware of the impact a formula (chemistry) can have on engineering materials.

• Maintenance and or plant engineers will be able to implement inspection programs to prevent unscheduled shutdowns due to corrosion.

Section 1 Overview & Corrosion Chemistry

Section 2 Forms of Corrosion

Section 3 Engineering Considerations

Presentation At A Glance

Agenda

Section 1: Introduction & Corrosion Chemistry

Approximate Time Slides

Welcome, Introductions 15 minutes 2

Corrosion Examples 15 minutes 8

Chemical Parameters 30 minutes 17

Passivation 15 minutes 8

Electrochemistry 10 minutes 3

Exercise 1-1: Chemistry 10 minutes 1

Review 10 minutes 1

Total Time 2 hours

BREAK

Agenda

Section 2 Corrosion Forms

Approximate Time Slides

General, Pitting and Crevice 30 minutes 22

Exercise 2-1 “Soft-soap” 25 minutes 5

Stress Cracking, Fatigue 15 minutes 15

Galvanic, Erosion, High Temp. 15 minutes 15

Exercise 2-2 “Sulfur burner Piping” 25 minutes 7

Review 10 minutes 1

Total Time 2 hours

LUNCH 1 hour

Agenda

Section 3 Engineering Considerations

Approximate Time Slides

Materials Selection and Testing 15 minutes 10

Equipment Design 30 minutes 18

Inspection for Corrosion-Process Checkpoints 15 minutes 8

Exercise 3-1: Manufacturing Corrosion Exercise 30 minutes 1

Formulating to Avoid Corrosion Tendencies Break

10 minutes 3

Engineering Materials – 12 Critical Points 30 minutes 14

Database Resources 10 minutes 4

Review 10 minutes 1

Total Time 3 hours

Wrap-Up

Business Impact Corrosion Examples

Rotary Valve

Picture of a valve that corroded after one week of service. The valve is made out of a martensitic stainless steel. The exact specification is 440B. This alloy was selected based on its wear properties.

Showing the severe localized attack

Close-up of Rotary Valve

Pump Springs

Soft-soap pump springs experienced corrosion of 302 SS in a matter of weeks that could be seen by the consumer. The corrosion occurred mainly on spring parts that were in contact with the plastic.

Could have changed a clear product to a cloudy one on the store shelf.

INTERNAL DEFORMATION

Caused by inner liner becoming brittle and welds failing with pressure.

Chemical Processing Column 304 SS 10 years Stress Corrosion

Drive Shaft Inline Draft Tube Mixer

9 years stress corrosion cracking

Mixer shaft key to production

Low pH Speed Stick Gel Formula

• Aluminum foil seal has an adhesive inner surface

• The gel formula attacks the adhesive and the aluminum.

• Example of packaging seal corrosion

Sulfur Burner Exhaust Piping

High temperature corrosion of 321 SS at 1300 F with SO2 after less than 2 years of service. This could pose a significant safety issue.

Metallurgical Review of Corrosion Issues

Where Chrome Nickel Carbon Molybdenum Implication

304 18-20 8-10.5 0.15

304L Columns 18-20 8-12 0.03 Supply chain

316L Draft tube mixer 16-18 10-14 0.03 2.0-3.0 Supply chain

440B Rotary valve 16-18 0.75 0.75 Product quality

321 Sulfur exhaust 17-19 9-12 0.08 Ti Safety EOHS

302 Soft-soap spring 17-19 8-10 0.15 Consumer

Chemical Parameters

Corrosion Basics-Chemistry

Corrosion

Definition:

Corrosion is the deterioration of a substance (usually a metal) or its properties because of a reaction with its environment.

Elements of a Corrosion Cell in Metals

Cathode

Anode

Electrolyte

Electrical Path

The Corrosion Cell

• Metallic corrosion is an “electrochemical reaction” which involves :

– a transfer of electrons

– oxidation - loss of electrons (corrosion)

– reduction - gain of electrons (protection)

– migration of ions

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++

e-

e- e-

e-

e-e-

e-

e-

e-

e-

e-

e-

e-e-

e-

e-e-

e-

ANODE

ELECTROLYTE

Anodic Process

Oxidation Reaction

H+

e-

e-

e-

e-

e-

e-

e- e-

e-

e-

e-e-

e-

e-

e-

e-

e-

CATHODEH

H+

H+

H+

H+

H+

HH

H+H+

H+H+

H+H+

H+

H

H

H

ELECTROLYTE

Cathodic Process

Reduction Reaction

Single Corrosion Cell

Fe++ Fe++

Fe++

Fe++

Fe++

OH-

OH-

OH-

Fe(OH)2Fe(OH)2Fe(OH)2H+

H+

H+

H+

H+

H+

H+

HHH

CATHODIC SITE ANODIC SITE

e-e-e-

e-

e-

Fe++

Microscopic View

e-

Cu

Fe

ee

ee

Electrical Path

CathodeAnode

Electrolyte

Fe++

H+

H+

H+OH—

H+OH—

Galvanic Corrosion

Electrochemical Cell Must Have:

• Anode - where oxidation occurs

• Cathode - where reduction occurs

• Electrolyte - where ion migration occurs

• Electrical path - where electrons migrate from the anode to the cathode

Example Carbon Steel in Varying pH

Anode: (Oxidation) Fe Fe++ + 2e

Cathode : (possible reduction reactions)

2H+ + 2e- H2 Acid pH

O2 + 4H+ + 4e- 2H2O Aerated Acid

O2 + 2H2O + 4e- 4OH- Alkaline pH

Electrolyte

H2O H+ + OH—

Effect of pH on Corrosion

• Acids - excess hydrogen ions H+

• Alkalis- excess hydroxyl ions OH-

• These ions are important drivers in oxidation and reduction reactions.

Effect of Low pH on Corrosion

In the electrolyte: HCl = H+ + Cl–

Anodic area on iron: Fe = Fe++ + 2e

Cathodic area: 2H+ + 2e = H2

Example: Iron Attack by Hydrochloric Acid

Effect of High pH on Materials

pH above 10

• Particular problems for metals are KOH, NaOH, and NH4 OH

• The main issue is metal embrittlement at high pH

• Nickel and nickel alloys best for strong NaOH

• “Caustic embrittlement”- Carbon Steel is an example of a material that can also be attacked at high pH

Au ---) Au +3 + 3e 1.420 Noble

Fe +3 + e ---) Fe +2 0.771

O2 + 2H20 + 4e ---) 40H- 0.410

Cu----) Cu+2 + 2e 0.340

2H+ + 2e ---) H2 0.000 Reference

Fe ---) Fe2+ + 2e -0.440

Zn ---) Zn+2 + 2e -0.760

Al ---) Al+3 + 3e -1.660 Active

Example Zn is corroded in Acids (H ion excess) and Cu will not as it is above reference Hydrogen ion reduction potential.

Another Driver of Corrosion Reactions are differences in Electrochemical PotentialsElectrochemical Potentials

Simplified Potential-pH Diagram

Fe +3Fe +3

Fe(OH)3Fe(OH)3

Fe +2Fe +2

Fe(OH)2Fe(OH)2

FeFe

pH

Potential

0 8 14

Reference Diagrams – Iron in Water

4

The Halogens

Effect

• Increase the conductivity of an electrolyte – ion migration

• Increase corrosion rate (localized pitting and crevice corrosion of stainless steels)

• Examples: Fluorine, Chlorine, Bromine

Temperature

As the temperature rises, the possibility of engineering material degradation increases significantly.

A Few Concentration and Reaction Rate Scenarios

• Increase linearly with concentration and then decrease dramatically. (Stainless steel in sulfuric acid)

• Begin only at higher concentrations and then increase exponentially. (Carbon steel in sodium hydroxide)

• No change in reaction rate as solution concentration changes. (316 SS in Nitric Acid)

Trace Compounds

• Corrosion can be controlled by as little as parts per million of an ion or ionized compound.

• For example – Chlorate (Cl03-), Fe+3, Cu+2

• Therefore it is important to report all parts of a process formula to predict the overall effect on a manufacturing plant.

Passivation and Stainless Steel

“The main objective of Passivation is to create a surface layer on stainless steel

that is more resistant to corrosion .”

Basic shapes

PLATE

BAR

Hot working

Forging

INGOT

(COOLING)

Charge Iron Chrome

Nickel balls

2400-2700 ºF

Drawing

Cold Working

REFINING

Stainless Steel Making Process

Stainless Steel MakingBASIC

SHAPES

PICKLINGHeat Treatment

(Depends on type of SS)

PASSIVATINGCHROMIUM OXIDE PASSIVE LAYER

Nitric Acid

Sulfuric Acid

Many Different materials with varying Chemical Resistance &

Mechanical properties

Various Stainless steel chemistries

Specified Heat Treatments+

Basic Principle

Effects of Surface Condition• 1 Sandblasted and

passivated

• 2 Electrolytically polished passivated

• 3 Ground and passivated

• 4 Pickled and passivated

• 5 Mechanically polished and passivated

• The higher the number the better the resistance to corrosion.

0

50

100

150

200

250

300

350

400

1 2 3 4 5

PittingPotential(mv)

Passive Layer – Chromium Oxide Film

• The reaction of chromium with air or passivating solution of nitric acid produces a very thin layer of oxide called the chromium oxide “passive” film.

• A minimum of 11% chromium content is needed for this layer to form.

• The film strength, resistance to “corrosion” oxidation, varies with the amount of chromium and the heat treated condition of the material, along with other factors.

Improving Stainless Steel Passivity

• To remove free Iron, points of future corrosion, utilize a 10% Citric Acid solution in water and circulate at 65 C (150 F) for 30 minutes throughout the plant.

• Free Iron sites are spots where corrosion can get started. This can lead to localize pits on the surface of the stainless steel and therefore sites for microbial growth.

Field Passivation (continued)

• To remove tightly adherent oxide films from welding- Utilize 10-15% Nitric Acid/ 1-3% Hydrofluoric solution at room temperature. Use a commercially available pickling gel.

• For localized treatments only and not for general use. Safety precautions need to be followed when working with strong chemicals.

Electrochemistry

Electrochemical Measurements

Stainless Steel Electrochemical Corrosion Diagram

Potential (mV)

Corrosion Current (mA)

Pitting

Passivity

General Corrosion

Chromium Oxide Layer

Pickling

Passivation

Not recommended alloy for Oral Care

Large passive region

316 SS 440B SS

Small passive region

General corrosion

Corrosion Diagrams for Two Different Types of Stainless Steel in Nitric Acid

316 440B

Pitting Unlikely Pitting Most Likely

Passivation Curves for Two Different Stainless Steels under Oxidizing Conditions

Large passive regionSmall passive region

Exercise 1-1 Corrosion Chemistry1. What are the four elements of a corrosion cell?

2. What are the two electrochemical components of a corrosion reaction?

3. What are the major inputs or variables that need to be accounted for in making a judgment on a engineering material?

4. What is the difference between pickling and passivation?

5. What is the benefit of using citric acid to clean stainless steel surfaces?

What Did We Learn

• Corrosion is a broad term defined here to include the deterioration (reaction) of any engineering material or structure with its environment.

• That corrosion if left unaccounted for can cause serious business issues.

• In the case of metals, corrosion is an electrochemical process.

• Stainless steel owes its chemical resistance to the chromium oxide passive film.

• The chemical resistance of the passive film depends on many factors.

• What the important inputs parameters are for selecting or deciding on an engineering material.

• How we can remove free Iron from stainless steel to prevent localized corrosion and thereby prevent future microbiological action at those sites.